The present invention relates to the structure of a recording medium for use with an electrophotographic copier, facsimile apparatus, printer or similar recorder.
A recorder operable with an electrostatic recording medium in the form of a sheet is conventional. The electrostatic recording medium has at least a conductive layer and a dielectric layer formed on the conductive layer. The recorder electrostatically records a latent image on the dielectric layer of the recording medium and then develops it by a developing device to produce a visible image. With this kind of recorder, it is necessary to maintain the conductive layer at a predetermined potential allowing the conductive layer to play the role of an electrode, so that the resulting images may remain stable. To meet this requirement, it has been customary to implement the conductive layer by a substrate which is infiltrated with a conductive agent at both sides thereof, the dielectric layer being formed on the conductive layer. The conductive substrate is connected to ground via the recorder. This kind of recording medium transmits light partly because it is infiltrated with a conductive agent and partly because its substrate is thin. Hence, such a recording medium is not a desirable material except as a second original document. For example, to achieve a high quality full-color image, the recording medium should preferably be as white and glossy as photographic paper. While opacity, gloss, whiteness and smoothness are achievable if the substrate is implemented as a film of polyethylene phtalate or similar synthetic resin or synthetic paper, it is difficult to reduce the resistance of such a film or paper.
To promote accurate and dense recording, there is a demand for a film of synthetic resin or similar insulative substrate which is not susceptible to changes in ambient conditions. There has been proposed a recording medium which is a laminate of a dielectric layer, an insulative substrate such as a film of synthetic resin, and a conductive layer intervening between the dielectric layer and the substrate. The problem with such a laminate or recording medium is that the substrate which is insulative cannot be directly grounded. Some different approaches to connect the insulative substrate to ground have been proposed, as follows:
(1) omitting the dielectric layer over several millimeters from the widthwise edge so as to expose the conductive layer to the outside; PA0 (2) applying a conductive agent which is a mixture of carbon black, metal powder or similar conductive particles and a resin to the so exposed portion to thereby form a conductive film; PA0 (3) breaking part of the dielectric layer and inserting a conductive projection into the recording medium at the broken part; and PA0 (4) mixing a small amount of conductive particles with the dielectric layer, in which case the connection to ground will be effected on the surface of the dielectric layer (e.g. Japanese Patent Laid-Open Publication Nos. 64647/1990 and 83547/1990).
However, the above-stated conventional approaches (1)-(3) have a drawback that part of the recording medium cannot be used to record an image thereon. Moreover, the approach (4) is likely to prevent a latent image from being formed in the area where the conductive particles exist or prevent a bias voltage from acting effectively in the developing device.